Curable fluoroelastomer composition

ABSTRACT

Fluoroelastomer compositions comprising fluoroelastomers having copolymerized units of a nitrile-containing cure site monomer are cured with certain hydrazone or oxime curatives.

FIELD OF THE INVENTION

This invention relates to curable fluoroelastomer compositions and moreparticularly to fluoroelastomer compositions containing certainhydrazones or oximes as curing agents.

BACKGROUND OF THE INVENTION

Fluoroelastomers have achieved outstanding commercial success and areused in a wide variety of applications in which severe environments areencountered, in particular those end uses where exposure to hightemperatures and aggressive chemicals occurs. For example, thesepolymers are often used in seals for aircraft engines, in oil-welldrilling devices, and in sealing elements for industrial equipment thatoperates at high temperatures.

The outstanding properties of fluoroelastomers are largely attributableto the stability and inertness of the copolymerized fluorinated monomerunits that make up the major portion of the polymer backbones in thesecompositions. Such monomers include vinylidene fluoride,hexafluoropropylene, tetrafluoroethylene and perfluoro(alkyl vinyl)ethers. In order to develop elastomeric properties fully,fluoroelastomers are typically crosslinked, i.e. vulcanized. To thisend, a small percentage of cure site monomer is copolymerized with thefluorinated monomer units. Cure site monomers containing at least onenitrile group, for exampleperfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene, are especially preferred.Such compositions are described in U.S. Pat. Nos. 4,281,092; 4,394,489;5,789,489; and 5,789,509.

Bisamidoximes (U.S. Pat. No. 5,668,221) and bisamidrazones (U.S. Pat.Nos. 5,605,973; 5,637,648) have been used as vulcanizing agents forfluoroelastomers having nitrile group cure sites. These cures may bescorchy, i.e. crosslinking may begin before the final shaping of thecomposition. Also, the curatives require complex, multistep synthesesfrom expensive starting materials.

Other nitrogen containing nucleophilic compounds have been employed tocrosslink fluoroelastomers having nitrile group cure sites (U.S. Pat.No. 6,638,999 B2). Some of these curatives are scorchy while others arevolatile at rubber milling temperatures.

Fluoropolymers having pendant amidrazone or amidoxime groups are alsoknown (U.S. Pat. No. 7,300,985 B2). These polymers require an additionalpolymer modification step in order to form crosslinks.

Azo and amide based composition are provided in U.S. Pat. No. 9,169,339.(Daikin Industries Ltd.) which include a carbonyl group proximal to thenitrogen-nitrogen bond.

There remains a need in the industry for curable composition whichprovide compression set at very high temperatures in multipleenvironments.

SUMMARY OF THE INVENTION

The present invention is directed to curable fluoroelastomercompositions which comprise a fluoroelastomer having nitrile group curesites and certain hydrazones or oximes as curatives. More specifically,the present invention is directed to a curable composition comprising:

A) a fluoroelastomer comprising copolymerized units of a nitrilegroup-containing cure site monomer; and

B) a hydrazone or oxime curing agent of the general formulaR¹(C═NY)_(n)R² wherein n=1 or 2, Y═OH, or NH₂, R¹ is aryl, alkyl,benzyl, perfluoroalkyl, or heterocyclic and R² is aryl or heterocyclic.

Another aspect of the present invention is a cured article made from theabove composition.

DETAILED DESCRIPTION OF THE INVENTION

Described herein are curing agents in compounds described herein. Alsodescribed herein are articles that have been cured and which, beforecuring, comprised such compounds. Also, described herein are processesfor curing the compounds described herein.

The fluoroelastomer that may be employed in the composition of theinvention may be partially fluorinated or perfluorinated.Fluoroelastomers preferably contain between 25 and 70 weight percent,based on the total weight of the fluoroelastomer, of copolymerized unitsof a first monomer which may be vinylidene fluoride (VF2) ortetrafluoroethylene (TFE). The remaining units in the fluoroelastomersare comprised of one or more additional copolymerized monomers,different from said first monomer, selected from the group consisting offluoromonomers, hydrocarbon olefins and mixtures thereof. Fluoromonomersinclude fluorine-containing olefins and fluorine-containing vinylethers.

Fluorine-containing olefins which may be employed to makefluoroelastomers include, but are not limited to vinylidene fluoride(VF2), hexafluoropropylene (HFP), tetrafluoroethylene (TFE),1,2,3,3,3-pentafluoropropene (1-HPFP), 1,1,3,3,3-pentafluoropropene(2-HPFP), chlorotrifluoroethylene (CTFE) and vinyl fluoride.

Fluorine-containing vinyl ethers that may be employed to makefluoroelastomers include, but are not limited to perfluoro(alkyl vinyl)ethers. Perfluoro(alkyl vinyl) ethers (PAVE) suitable for use asmonomers include those of the formulaCF₂═CFO(R_(f′)O)_(n)(R_(f″)O)_(m)R_(f)  (I)where R_(f′) and R_(f″) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R_(f) is a perfluoroalkyl group of 1-6 carbon atoms.

A preferred class of perfluoro(alkyl vinyl) ethers includes compositionsof the formulaCF₂═CFO(CF₂CFXO)_(n)R_(f)  (II)where X is F or CF₃, n is 0-5, and R_(f) is a perfluoroalkyl group of1-6 carbon atoms.

A most preferred class of perfluoro(alkyl vinyl) ethers includes thoseethers wherein n is 0 or 1 and R_(f) contains 1-3 carbon atoms. Examplesof such perfluorinated ethers include perfluoro(methyl vinyl ether)(PMVE), perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinylether) (PPVE). Other useful monomers include those of the formulaCF₂═CFO[(CF₂)_(m)CF₂CFZO]_(n)R_(f)  (III)where R_(f) is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1,n=0-5, and Z═F or CF₃. Preferred members of this class are those inwhich R_(f) is C₃F₇, m=0, and n=1.

Additional perfluoro(alkyl vinyl) ether monomers include compounds ofthe formulaCF₂═CFO[(CF₂CF{CF₃}O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)  (IV)where m and n independently=0-10, p=0-3, and x=1-5. Preferred members ofthis class include compounds where n=0-1, m=0-1, and x=1.

Other examples of useful perfluoro(alkyl vinyl ethers) includeCF₂═CFOCF₂CF(CF₃)O(CF₂O)_(m)C_(n)F_(2n+1)  (V)where n=1-5, m=1-3, and where, preferably, n=1.

If copolymerized units of PAVE are present in fluoroelastomers employedin the invention, the PAVE content generally ranges from 25 to 75 weightpercent, based on the total weight of the fluoroelastomer. Ifperfluoro(methyl vinyl ether) is used, then the fluoroelastomerpreferably contains between 30 and 65 wt. % copolymerized PMVE units.

Hydrocarbon olefins useful in the fluoroelastomers employed in theinvention include, but are not limited to ethylene and propylene. Ifcopolymerized units of a hydrocarbon olefin are present in thefluoroelastomers, hydrocarbon olefin content is generally 4 to 30 weightpercent.

The fluoroelastomer further contains copolymerized units of at least onecure site monomer, generally in amounts of from 0.1-5 mole percent. Therange is preferably between 0.3-1.5 mole percent. Although more than onetype of cure site monomer may be present, most commonly one cure sitemonomer is used and it contains at least one nitrile substituent group.Suitable cure site monomers include nitrile-containing fluorinatedolefins and nitrile-containing fluorinated vinyl ethers. Usefulnitrile-containing cure site monomers include those of the formulasshown below.CF₂═CF—O(CF₂)_(n)—CN  (VI)where n=2-12, preferably 2-6;CF₂═CF—O[CF₂—CFCF₃—O]_(n)—CF₂—CFCF₃—CN  (VII)where n=0-4, preferably 0-2;CF₂═CF—[OCF₂CFCF₃]_(x)—O—(CF₂)_(n)—CN  (VIII)where x=1-2, and n=1-4; andCF₂═CF—O—(CF₂)_(n)—O—CF(CF₃)CN  (IX)where n=2-4.Those of formula (VIII) are preferred. Especially preferred cure sitemonomers are perfluorinated polyethers having a nitrile group and atrifluorovinyl ether group. A most preferred cure site monomer isCF₂═CFOCF₂CF(CF₃)OCF₂CF₂CN  (X)i.e. perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) or 8-CNVE.

A first aspect of this invention is a curable composition comprising afluoroelastomer comprising copolymerized units of a cure site monomerselected from the group consisting of nitrile-containing fluorinatedolefins and nitrile-containing fluorinated vinyl ethers; and a certainhydrazone or oxime. The hydrazones or oximes may be a mono- or di-,hydrazones or oximes and included as curing agents.

The hydrazones or oximes are of the general formula R¹(C═NY)_(n)R²,wherein n=1 or 2, Y═OH, or NH₂, R¹ is aryl, alkyl, perfluoroalkyl, orheterocyclic, and R² is aryl or heterocyclic. When R¹ and R² are botharyl, they may be fused such as a fluoroenyl group (see Example 3herein). The alkyl, aryl, benzyl, perfluroakyl or heterocycle groups maycontain additional functional groups such as, but not limited tohalogen, ether, or amide groups. The perfluoroalkyl group has at leastone of the hydrogen atoms replaced by fluorine. Compounds that decomposeto form one of these hydrazones or oximes may also be employed in thecomposition of the invention. The hydrazones or oximes may also be asalt, e.g. hydrochloride.

Preferred hydrazones or oximes include those wherein n is 1; R¹ is aryland R² is aryl. A most preferred curative composition includes curingagent wherein n=2 and the curing agent is C₆H₅(C═NNH₂)₂C₆H₅.

It is theorized that these hydrazones of the present invention act ascuring agents by causing the dimerization of polymer chain bound nitrilegroups to form 1,2,4-triazole rings, thus crosslinking thefluoroelastomer. The hydrazones of the present invention are lessvolatile than curatives such as hydrazine or t-butyl carbazate, makingthe curatives less likely to be fugitive during mixing and shapingprocesses.

In order to be useful as either the major, or as the only curative forthese fluoroelastomers, the level of hydrazones or oximes should beabout 0.05 to 7 parts hydrazone or oxime per 100 parts fluoroelastomer,preferably about 0.1 to 3 parts hydrazone or oxime per 100 partsfluoroelastomer, most preferably about 0.5 to 2 parts hydrazone or oximeper 100 parts fluoroelastomer. As used herein, “parts” refers to partsby weight, unless otherwise indicated.

An appropriate level of hydrazone or oxime can be selected byconsidering cure properties, for example the time to develop maximummoving die rheometer (MDR) torque and minimum Mooney scorch of thecurable compositions. The optimum level will depend on the particularcombination of fluoroelastomer and hydrazone or oxime.

Optionally, a curative accelerator, e.g. a compound that releasesammonia at curing temperatures, may be used in combination with anhydrazone or oxime curative. Examples of compounds that decompose torelease ammonia at curing temperatures include those disclosed in U.S.Pat. No. 6,281,296 B1 and U.S. 2011/0009569.

Optionally, another curative commonly employed to crosslinkfluoroelastomers having nitrile-group cure sites may be used in additionto the hydrazones or oximes. Examples of such other curatives include,but are not limited to diaminobisphenol AF,2,2-bis(3-amino-4-anilinophenyl)hexafluoropropane, mono- orbis-amidines, mono- or bis-amidrazones, mono- or bis-amidoximes, or anorganic peroxide plus coagent.

Additives, such as carbon black, fluoropolymer micropowders,stabilizers, plasticizers, lubricants, fillers, and processing aidstypically utilized in fluoroelastomer compounding can be incorporatedinto the compositions of the present invention, provided they haveadequate stability for the intended service conditions.

The curable compositions of the invention may be prepared by mixing thefluoroelastomer, hydrazones or oximes and other components usingstandard rubber compounding procedures. For example, the components maybe mixed on a two roll rubber mill, in an internal mixer, for example, aBanbury® internal mixer, or in an extruder. The curable compositions maythen be crosslinked (i.e. cured) by application of heat and/or pressure.When compression molding is utilized, a press cure cycle is generallyfollowed by a post cure cycle during which the press cured compositionis heated at elevated temperatures in excess of 300° C. for severalhours.

The hydrazone curable compositions of the present invention are usefulin production of cured articles such as gaskets, tubing, and seals. Thecured articles are generally produced by molding a compoundedformulation of the curable composition with various additives underpressure, curing the part, and then subjecting it to a post cure cycle.The cured compositions have excellent thermal stability, steam andchemical resistance. Volume swell (ASTM D1414) after exposure to 225° C.water for at least 168 hours, preferably at least 336 hours, is lessthan 1%. Also compression set, 300° C., 70 hours, 15% compression (ASTMD395) is less than 70%. The cured compositions are particularly usefulin applications such as seals and gaskets for manufacturingsemiconductor devices, and in seals for high temperature automotiveuses.

Other fluoropolymers containing nitrile cure sites, such asfluoroplastics may be substituted for fluoroelastomers in thecompositions of the invention.

The invention is now illustrated by certain embodiments wherein allparts are by weight unless otherwise specified.

EXAMPLES Test Methods Cure Characteristics

Cure characteristics were measured using a Monsanto MDR 2000 instrumentunder the following conditions:

-   -   Moving die frequency: 1.66 Hz    -   Oscillation amplitude: ±0.5 degrees    -   Temperature: 190° C., unless otherwise noted    -   Sample size: Disks having diameter of 1.5 inches (38 mm)    -   Duration of test: 30 minutes

The following cure parameters were recorded:

-   -   M_(H): maximum torque level, in units of dN·m    -   M_(L): minimum torque level, in units of dN·m    -   Tc90: time to 90% of maximum torque, minutes

Test specimens were prepared from elastomer compounded with appropriateadditives, as described in the formulations listed in the Examplesbelow. Compounding was carried out on a rubber mill. The milledcomposition was formed into a sheet and a 10 g sample was die cut into adisk to form the test specimen.

Compression set of O-ring samples was determined in accordance with ASTMD395. Mean values are reported.

Volume swell in water was measured at 225° C. for the time indicated inthe Tables in accordance with ASTM D1414.

The following fluoroelastomer polymers were used in the Examples:

-   FFKM—A terpolymer containing 61.8 mole percent units of TFE, 37.4    mole percent units of PMVE and 0.80 mole percent units of 8-CNVE was    prepared according to the general process described in U.S. Pat. No.    5,789,489.

Examples 1-3

Curable compositions of the invention were compounded on a two-rollrubber mill in the proportions shown in Table I. The compoundedcompositions are labeled Example 1 (Benzil dihydrazone, CAS #4702-78-7,available from Sigma-Aldrich), Example 2 (Benzophenone hydrazone, CAS#5350-57-2, available from Sigma-Aldrich) and Example 3 (9-fluorenonehydrazone, CAS #13629-22-6, available from Alfa-Aesar) in Table I. Curecharacteristics of the compounded compositions are also shown in TableI. O-rings were made by press curing the curable compositions at atemperature of 190° C. for Tc90 plus 5 minutes, followed by a post curein a nitrogen atmosphere at a temperature of 305° C. for 26 hours aftera slow temperature ramp up from room temperature. Compression set andvolume swell values are also shown in Table I.

TABLE I Formulation (phr¹) Example 1 Example 2 Example 3 FFKM -Polymer100 100 100 Carbon Black MT N990 30 30 30 Benzil dihydrazone 1.0 0 0(CAS # 4702-78-7) Benzophenone hydrazone 0 1.7 0 (CAS # 5350-57-2)9-fluorenone hydrazone 0 0 1.5 (CAS # 13629-22-6) Cure Characteristics @190° C. M_(L) (dN · m) 3.36 2.56 2.36 M_(H) (dN · m) 20.5 12.6 16.67Tc90, minutes 20.1 15.8 15.1 Compression set, % in distilled 73 78 76water at 250° C., 70 hours compression, 15% Compression set, %, indistilled 74 82 80 water at 225° C., 168 hours compression, 15%Compression set, 300° C., 70 hours, 33 19 28 compression, 15% Volumeswell, % in distilled 0.32 0.36 0.91 water at 225° C., 168 hours ¹Partsby weight per hundred parts by weight perfluoroelastomer

Comparative Examples 1 and 2

Compositions of the invention were compounded on a two-roll rubber millin the proportions shown in Table II. The compounded compositions arelabeled Comparative Examples 1 and 2 (benzil monohydrazone, CAS#5344-88-7, available from Acros Organics), Cure characteristics of thecompounded compositions are also shown in Table II.

TABLE II Comparative Comparative Formulation (phr) Example 1 Example 2Example 4 FFKM 100 100 100 Carbon Black MT N990 30 30 50 Benzilmonohydrazone 1.0 2.0 0 (CAS # 5344-88-7) Benzophenone oxime 0 0 1.35(CAS # 574-66-3) Cure Characteristics @ 190° C. @ 190° C. @ 200° C.M_(L) (dN · m) 1.97 1.81 4.59 M_(H) (dN · m) 3.47 4.41 6.9 Tc90, minutes25 25 13.98

As illustrated by Comparative Examples 1 and 2, a curative with acarbonyl group adjacent to hydrazone group is not an effectivecrosslinking agent as evident from the very low M_(H) (dN·m) value inTable II.

What is claimed is:
 1. A curable composition comprising: A) afluoroelastomer comprising copolymerized units of a nitrilegroup-containing cure site monomer; and B) a hydrazone or oxime of thegeneral formula R¹(C═NY)_(n)R² wherein n=1 or 2, Y═OH, or NH₂, R¹ isaryl, alkyl, perfluoroalkyl, benzyl or heterocyclic and R² is aryl orheterocyclic.
 2. The curable composition of claim 1, wherein n is 1; R¹is aryl, R² is aryl and Y is NH₂.
 3. The curable composition of claim 1,wherein n is 1; R¹ is aryl, R² is aryl and Y is OH.
 4. The curablecomposition of claim 1 wherein n is 1; R¹ is aryl or heterocyclic, R² isaryl and Y is NH_(2.)
 5. The curable composition of claim 1 wherein n=2and the hydrazone is C₆H₅(C═NNH₂)₂C₆H₅.
 6. The curable composition ofclaim 1, wherein R¹ and R² are aryl and fused together to form afluoroenyl, and Y is NH₂.
 7. The curable composition of claim 1, whereinan appropriate level of hydrazone or oxime is selected by the timenecessary to develop maximum moving die rheometer (MDR) torque andminimum Mooney scorch of the curable composition.
 8. The curablecomposition of claim 7, wherein a carbonyl group is not adjacent to thehydrazone or oxime.
 9. A cured article made from the composition ofclaim
 1. 10. A cured article made from the composition of claim
 8. 11. Acurable composition consisting of: A) a fluoroelastomer comprisingcopolymerized units of a nitrile group-containing cure site monomer; andB) a hydrazone of the general formula R¹(C═NNH₂)_(n)R² wherein n=1 or 2,and R¹ is aryl, alkyl, perfluoroalkyl, or heterocyclic and R² is aryl orheterocyclic.
 12. A cured article made from the composition of claim 11having a volume swell, measured according to ASTM D1414, after exposureto 225° C. water for at least 168 hours of less than 1% and acompression set, 300° C., 70 hours, 15% compression, measured accordingto ASTM D395, of less than 70%.